Analysis Of Spatiotemporal Variation And Influencing Factor Of Net Primary Productivity In Nanjing

Forest is the largest carbon pool in terrestrial ecosystems,and urban forest is an important part of urban ecosystems.Forest converts large amounts of CO₂ into organic carbon through photosynthesis and store it in plants,Therefore,the study of NPP in urban forest is great theoretical and practical importance in mitigating the rising concentration of greenhouse gases such as CO₂ in the urban atmosphere and maintaining the health and stability of ecosystems.Based on remote sensing image datasets,meteorological data and forest vegetation types,the optimized CASA model was used to estimate the forest NPP in Nanjing from 2000 to 2019,and the Mann-Kendall test and Sen-Median trend analysis method and optimal parameters-based geographical detector were used to explore the spatiotemporal variation and driving forces of forest NPP in Nanjing from 2000 to 2019.To provide a scientific data base for the effective management of urban forest ecosystems and carbon pools in the context of urbanization in China,and to provide a scientific basis for regional ecosystem quality evaluation and urban ecological environmental protection planning.The main conclusions are as follows:（1）The NPP of Nanjing forest estimated based on the CASA model was verified with the accuracy of existing NPP data,and the R² was 0.68 and above.（2）The results of the analysis of the temporal variation characteristics of forest NPP in Nanjing showed that in the interannual variation,forest NPP in Nanjing showed an increasing trend with time,but the increasing trend was not significant,among which the proportion of the areas with increasing and decreasing trend of forest NPP were 65.19%and 34.81%,respectively;in the seasonal variation,spring was the season with the fastest growth of forest NPP,and summer was the forest NPP accumulation period with the most.In the inter-month variation,the distribution of the mean value of forest vegetation NPP in each month showed that April to September is the accumulation period of NPP,and July and August were the highest period of forest NPP accumulation.（3）From the spatial distribution and change trend of forest NPP,the overall distribution pattern of forest NPP in Nanjing over the past 20 years was"low in the north and south,high in the middle".The forest NPP range in Nanjing from 2000 to 2019 was 1.09 to 1700.192)?8)^-2?(6^-1,with a multi-year average of 476.472)?8)^-2?(6^-1.Total annual average forest NPP was386×10⁹2)?(6^-1.The coefficient of variation of forest NPP ranged from 0.06 to 0.76,with a mean value of 0.13,and the overall forest NPP in Nanjing was in a medium fluctuation state;the Hurst exponent of forest NPP ranged from 0.16 to 0.85,with a mean value of 0.49,and the future trend of most of the forest NPP in Nanjing was consistent with the past,showing a persistent and mainly weakly increasing change.（4）The results of the analysis of forest NPP drivers in Nanjing showed that the explanatory power of different factors is different,in descending order:NDVI>forest vegetation type>forest vegetation coverage>elevation>enhanced vegetation index>evapotranspiration>slope>soil type>mean annual temperature>total annual precipitation>total solar radiation>slope direction.The explanatory power after the interaction of the two factors was greater than that of the single factor,and both showed non-linear enhancement and two-factor enhancement;the maximum value was NDVI?forest vegetation type=0.9392,which showed two-factor enhancement,and the minimum value was annual mean temperature?total solar radiation=0.1842,which showed non-linear enhancement.